This invention relates to semiconductor packages, and more particularly to a semiconductor package having a die pad for a semiconductor chip to be mounted thereon.
Semiconductor devices are required to be miniaturized in profile in order to cope with the recent tendency of reduction in size and weight of electronic equipments. As a result thin type semiconductor devices, such as TSOP (Thin Small Outline Package), SSOP (Shrink Small Outline Package), and TQFP (Thin Quad Flat Package), are developed to conform to industrial needs.
Illustrated in FIG. 1A is a cross-sectional view of a conventional thin type semiconductor package in the mold injection process. The laminated structure of a die 1 and a die pad 3 on which the die 1 is mounted, is received within a mold cavity 71 of a conventional encapsulating mold 7 composed of an upper mold 4 and a lower mold 5. Melted encapsulating resin 2 is introduced via a gate 6 of the encapsulating mold 7 into the mold cavity 71 and is thereby divided into an upper resin flow 24 passing through the space above the die 1 and a lower resin flow 25 passing through the space below the die pad 3. When the upper resin flow 24 moves at a speed substantially equivalent to the lower resin flow 25, the laminated structure of the die 1 and die pad 3 is kept horizontally stable in the mold cavity 71 until the encapsulating resin 2 completely fills the mold cavity 71, as shown in FIG. 18.
However, as the size of the chip chances, the velocity of the upper resin flow 24 would differ from that of the lower resin flow 25. This tends to incline the laminated structure of the die 1 and die pad 3 and thereby results in an adverse effect on the reliability of a semiconductor device. Referring to FIG. 2A, while chip 1xe2x80x2 is of a size smaller than the chip 1 of regular size as shown in FIG. 1A, the upper resin flow 24 moves faster than the lower resin flow 25 and incurs pressure against the chip 1xe2x80x2 by the upper resin flow 24 greater than that against the die pad 3 by the lower resin flow 25. As a result, the laminated structure of the die 1xe2x80x2 and die pad 3 downwardly inclines toward the inner surface 51 of the lower mold 5, so that the die pad 3 will be exposed to the outside of the encapsulant formed by the encapsulating resin 2 at the time the transfer molding process is completed, as shown in FIG. 2B.
Referring to FIG. 3A, while chip 1xe2x80x3 is of a size larger an the chip 1 of regular size as shown in FIG. 1A, contrary to what has been described above, the upper resin flow 24 moves slower than the lower resin flow 25 and incurs a pressure against the chip 1xe2x80x3 by the upper resin flow 24 smaller than that against the die pad 3 by the lower resin flow 25. As a result, the laminated structure of the chip 1xe2x80x3 and die pad 3 inclines upwardly toward the inner surface 41 of the upper mold 4, so that the die 1xe2x80x3 and/or gold wires (not shown) for electrically connecting the chip 1xe2x80x3 and inner leads (not shown) are exposed to the outside of the encapsulant formed by the encapsulating resin 2 at the time the transfer molding process is completed, as shown in FIG. 3B.
Therefore, to avoid the aforesaid exposures of the die and die pad to the encapsulant from taking place, U.S. Pat. No. 5,371,044 discloses a process for preparing a semiconductor device in which a control plate is used in the molding process to prevent the resin flows above the die and below the die pad from being different in velocity. As shown in FIG. 4, in the U.S. Pat. No. 5,371,044, a control plate 8, which projects inwardly toward the die pad 3 and horizontally extends from the inner edge of a lead frame (not shown), is formed near the gate 6 of the encapsulating mold 7 to balance the velocity of the upper resin flow 24 and that of the lower resin flow 25. However, the control plate 8 is only designed to decrease the velocity of the upper resin flow 24 so that it is substantially equal to that of the lower resin flow 25 where the upper resin flow 24 moves faster than the lower resin flow 25 in transfer molding. Thus, while the size of the chip 1 is increased to cause the lower resin flow 25 to move faster than the upper resin flow 24, the control plate is useless for balancing the upper resin flow 24 and the lower resin flow 25 such that in this case the chip 1 and die pad 3 tend to upwardly incline and are exposed to the encapsulant. Therefore, the use of the control plate of the U.S. Pat. No. 5,371,044 can not effectively solve the problem caused by the velocity unbalance between the upper resin flow and the lower resin flow in the transfer molding process.
Accordingly, as shown in FIG. 5, the U.S. Pat No. 5,623,162 discloses a die pad 3xe2x80x2 having a pair of wing leads 8xe2x80x2 which outwardly end from a pair of opposing sides of the die pad 3xe2x80x2 to the spacing between a pair of adjacent leads 89xe2x80x2. The wing leads 3xe2x80x2 are slightly projected from the side surface of the encapsulant 9xe2x80x2, as shown in FIG. 6, after the transfer molding process is completed in the transfer molding process, the outer end of the wing lead 8xe2x80x2 is sandwiched between the upper mold and the lower mold (not shown) such that the die pad 3xe2x80x2 is held in position by means of the wing leads 8xe2x80x2. As a result, the die pad 3xe2x80x2 does not have upward or downward slant problem caused by the velocity unbalance between the upper resin flow and the lower resin flow as described above.
Although a die pad constructed with wing leads as disclosed in the U.S. Pat No. 5,623,162 is able to solve the positioning problem of the die pad, it is primarily designed to provide the moisture in the encapsulant with a path to expel the moisture from the encapsulant to the atmosphere while avoiding the crack of the encapsulant caused by the xe2x80x9cpopcornxe2x80x9d effect, but not to solve the velocity unbalance between the upper resin flow and the lower resin flow in the transfer molding process. Meanwhile, the trimming process for cutting the wing leads is different from the conventional lead cutting/trimming process. As a result, it becomes necessary to incorporate an additional wing lead trimming step to the packaging process and to use equipment different from conventional ones, whereby the manufacturing cost is undesirably increased. Further, the die pad formed with wing leads increases the material cost for the lead frame, resulting in the wing-lead design not being widely accepted in the industry.
The present invention was made in accordance with the circumstances of the prior art, and therefore the object of the present invention is to provide a semiconductor package having a die pad with downward-extended tabs which allows the die pad to be maintained in position and not to be exposed to the encapsulant in the transfer molding process without increasing the material cost and packaging cost and using additional treatment process and equipment.
The above and other objects of the present invention are achieved by a semiconductor package comprising a semiconductor chip, a die pad for bearing the semiconductor chip, a plurality of leads each having an inner lead electrically connected with electrode pads disposed on the semiconductor chip by bonding wires such as gold wires, and an encapsulant formed by a molding resin for encapsulant the semiconductor chip, the die pad, and the inner leads of the leads The die pad is formed with a plurality of tabs extending downwardly from the plane where the die pad is positioned, allowing a lower resin flow of the molding resin flowing below the die pad to move slower than an upper resin flow of the molding resin flowing above the die in the molding process. Thus, a downward pressure is produced to press the die pad with the die mounted thereon until the tab of the die pad reaches the inner source of a mold cavity of a mold for forming the encapsulant.
The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings. It is important to point out that the illustrations may not necessarily be drawn to scale, and that there may be other embodiments of the present invention that are not specifically illustrated.